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The esterase activity of rat liver was mainly concentrated in the microsomal and nuclear fractions, with low activities in other fractions. On the basis of sensitivity t o certain esterase inhibitors, the microsomal esterases were considered to be of a carboxylesterase type. The esterase distribution in sonicated rough microsomes, separated by zone centrifugation, was also investigated.Solubilization of the microsomal esterases was achieved by sodium deoxycholate, trypsin and phospholipase-A treatment. Two esterases were solubilized from rat-liver microsomes by phospholipase-A treatment and were further purified by Chromatography on Sephadex G-100 and DEAE-cellulose. The enzymes were purified 256 and 154 times. The isoelectric points, K , values and molecular weights of the two esterases were determined.Both enzymes are of a carboxylesterase type. One of the esterase, however, was sensitive to HgCl, of low concentrations, which is an unusual property for carboxylesterases. The question of whether the two enzymes constitute a monomer and a dimer of the same esterase or are completely different enzymes, is discussed, The cellular distribution of esterases in the livers of different species has been reported frequently. The microsomal fraction seems to be the main source of the carboxylesterases [l -41, where, as this paper will show, they are bound to the microsomal membrane.